When a continuous casting apparatus through electromagnetic induction to which a bottomless cold mold partly divided along a circumferential direction is disposed (hereinafter, referred to as “electromagnetic casting apparatus”) is used, a molten substance (molten silicon herein) and the mold are in almost no contact with each other; resulting in an ingot (silicon ingot) without an impurity contamination. Further, a significant reduction of production cost can be achieved because of an advantage that the apparatus does not require a highly-pure material as a material of the mold since there is no contamination from the mold and because a continuous casting can be performed. Therefore, the electromagnetic casting apparatus has conventionally been applied to the production of polycrystalline silicon used as a solar cell substrate material.
FIG. 6 is a view schematically showing a configuration example of an electromagnetic casting apparatus preferable to produce polycrystalline silicon. As shown in the figure, copper plate-shape elements elongated in a vertical direction of which interior can be cooled with water are arranged inside an induction heating coil 2 in parallel to the direction of an axis of the induction coil 2, each element being in a mutually-insulated state inside the induction coil 2. A space surrounded by these plate-shape elements constitutes a mold (that is, a bottomless cold mold of which side wall is cooled with water) 1. In general, a water cooling copper mold in which a plate-shape element is made of copper is used as the cold mold 1.
A support base 7, which is downwardly movable, is installed in a lower end position of the induction heating coil 2 (that is, a position corresponding to a bottom part of the cold mold 1). Below the induction heating coil 2, a heat retention device 5 for warming a solidified ingot (a silicon ingot) 8 so as to prevent abrupt cooling is installed. Below the heat retention device 5, a heat retention cylindrical body 9 is attached. The silicon ingot 8 is withdrawn downwardly by a withdrawing device (not shown).
Above the cold mold 1, a raw material feeder 10 capable of feeding a raw material into the mold 1 during melting is installed. Further, in this example, a heating member 11 for heating raw material silicon as necessary is attached above the mold 1. It is preferable for a plasma torch to be arranged as the heating member 11 so as to perform plasma arc heating as necessary.
These devices are installed in a sealed container 6 so that molten silicon 4 and the high-temperature silicon ingot 8 are not brought into direct contact with the atmosphere. In general, an interior of the container 6 can be replaced with an inert gas so as to perform continuous casting in a slightly pressurized state.
Upon production of polycrystalline silicon, when the silicon material is charged into the mold 1 and a high-frequency induction current is applied to the induction heating coil 2, the material is heated and melted. The molten silicon 4 in the mold 1 repels the plate-shape elements due to the induction current and is immune from contacting with a side wall of the mold 1. When the support base 7 is gradually moved downward after the molten silicon 4 is sufficiently homogenized, cooling is started as part of the molten silicon leaves away from the induction coil 2, unidirectional solidification for the relevant molten silicon 4 in the mold 1 is developed, and the silicon ingot 8 having the same sectional shape as that of the mold is formed.
In accordance with a downward movement amount of the support base 7, an amount of the molten silicon 4 is decreased. Thus, by compensating the same amount of the material silicon from the raw material feeder 10 so as to constantly maintain an upper surface of the molten silicon 4 at the same height level, and continuously performing heating and melting, withdrawing, and raw material supply; the polycrystalline silicon ingot 8 can continuously be produced.
In order to improve the quality of polycrystalline silicon produced using this electromagnetic casting apparatus, in particular, to enhance a conversion efficiency when polycrystalline silicon is used particularly as a solar cell substrate (a ratio of energy capable of being converted into electric energy and taken out relative to incident optical energy), a lot of technology developments have been conducted in the past.
For example, PATENT LITERATURE 1 discloses a casting method of polycrystalline silicon in which a frequency of an alternating current supplied to an induction coil is 25 to 35 kHz. According to the casting method described in the above document, by increasing the frequency of the alternating current, a skin effect is generated in molten silicon and current density on a surface is increased. Accordingly, a surface temperature of an ingot is maintained at a high temperature and start of solidification due to cooling from the surface is delayed, so that growth of a chill layer on the ingot surface (of which crystal grain size is small and on which many crystal faults exist, so that a semiconductor characteristic is not preferable) can be suppressed. Further, since a coil current can be lowered, an electromagnetic stirring force acting on the molten silicon can be reduced so as to suppress stirring of the molten silicon. As a result, the growth of a crystal having a large grain size is facilitated, so that the conversion efficiency as a solar cell can be improved.
PATENT LITERATURE 2 discloses a silicon continuous casting method in which both electromagnetic induction heating of an induction coil and plasma heating of a transferable plasma arc are used, so that the quality as a solar cell is improved. According to the casting method described in the same document, by using therewith the plasma heating for melting a material during casting, the load of the electromagnetic induction heating can be reduced, and by suppressing thermal convection of molten silicon due to an electromagnetic force and suppressing a downward heat flow rate, a solid-liquid interface is flattened. As a result, a temperature gradient in a radial direction of a silicon ingot immediately after solidification is reduced and thermal stress generated inside a crystal is mitigated, so that the generation of crystal defects, which deteriorates the conversion efficiency of a solar cell, is suppressed.
According to such a casting method through this electromagnetic induction, a polycrystalline silicon ingot having a high conversion efficiency when the ingot is used as a solar cell substrate can be produced. However, in an actual operation, at the time of final solidification at which casting ends, solidification is started from an upper surface of the molten silicon in the mold and the central portion of the molten silicon is solidified lastly. As a result, due to volume expansion of the solidified part, cracking is generated in a finally solidified part of the ingot and that part has to be cut off and removed as an unusable part, so that a yield ratio is lowered.
As a solution of this problem, PATENT LITERATURE 3 proposes a silicon continuous casting method in which, in the end of the casting, a heating member which generates heat by itself due to electromagnetic induction of an induction coil is disposed so as to face a silicon melt remaining in a bottomless crucible from above, thereby preventing the solidification of the silicon melt from an upper surface, during which the solidification of the silicon melt is finished. According to the casting method described in the same document, cracking in a solidified part of the remaining melt can be prevented, and the crystal directionality similar to the other parts is given to this part, so that a high quality is obtained over the entire ingot. Thus, the casting method delivers a great effect for improving a yield ratio of production of the silicon ingot.
However, in the silicon continuous casting method described in PATENT LITERATURE 3, a silicon material is melted only through electromagnetic induction heating, without plasma heating by means of plasma arc or the other heating methods. There is a need for confirmation for application to electromagnetic casting in which the plasma heating is used as an auxiliary heat source.
In the continuous casting method described in PATENT LITERATURE 3, as described in an embodiment thereof, sizes of the produced ingot are 85 mm square and 117 mm square. It is unknown whether the casting method can be applied without any problems to such a case where a large ingot in a square form having a side length of 300 mm or more is produced.